crypto/tls: always send a Certificate message if one was requested.
crypto/tls: always send a Certificate message if one was requested.

If a CertificateRequest is received we have to reply with a
Certificate message, even if we don't have a certificate to offer.

Fixes issue 3339.

R=golang-dev, r, ality
CC=golang-dev
http://codereview.appspot.com/5845067

gnanderson/go,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang